1. Field of the Invention
The present invention relates to a light-emitting organic compound (including a complex that contains a metal in its molecular formula) capable of providing Electro Luminescence (EL), and an EL display device utilizing the same. Typically, the present invention relates to a high-molecular type EL display device which utilizes a light-emitting organic compound made of a high-molecular compound.
The present invention also relates to an electronic apparatus including the above-mentioned EL display device as its display portion. It should be noted that the above-mentioned EL display device will be also referred to as the OLED (Organic Light-emitting Diode).
2. Description of the Related Art
Development of a display device including an EL layer as a self-light emitting element that utilizes EL phenomenon (i.e., the EL display device) has been proceeded in these years. Since the EL display device is of the self-light emitting type, no back light is required to be contained therein, unlike a liquid crystal display device or the like. Moreover, the EL display device exhibits a wide viewing angle. From the above features, the EL display device is advantageous to be used as a display portion for a portable device which is likely to be used outdoors.
A light-emitting layer as a principal portion of the EL element is made of an insulating material. When a voltage is applied across a cathode and an anode with the light-emitting layer interposed therebetween, carriers (electrons and holes) are injected into the light-emitting layer and recombined to emit light. Thus, a current flowing through the light-emitting layer is caused by the recombination of carriers. An EL material that can be used for the EL display device is described in, for example. Japanese Patent Application Laid-Open No. Hei 2-311591.
In a light-emitting element such as a light-emitting diode in which a semiconductor junction is formed, Na (sodium) that may exhibit an adverse effect as a movable ion causes a resistance value of the light-emitting layer to decrease, and therefore, can cause a current flow other than that caused by the carrier recombination. When such an unnecessary current flows, an amount of heat generation is increased and the light-emitting layer is more likely to deteriorate. The same disadvantage may occur in the EL device. However, any sufficient countermeasure against the disadvantage caused by the movable ion has not been provided for the EL material.
The present invention is intended to provide an EL display device with high reliability. The present invention is also intended to provide an electronic apparatus with a highly reliable display portion by utilizing such an EL display device as its display portion.
In accordance with the present invention, in order to prevent a current from flowing due to reasons other than the carrier recombination, a volume resistivity of a thin film made of a light-emitting organic compound in an EL device is set to be in the range of 3xc3x971010 xcexa9cm or larger. A volume resistivity of a thin film made of a light-emitting organic compound in an EL device is set to be in the range from 1xc3x971011 to 1xc3x971012 xcexa9cm (preferably, in the range from 1xc3x971012 to 1xc3x971013 xcexa9cm). In order to obtain the volume resistivity value in the above range, the concentration of ionic impurities contained in the thin film made of the light-emitting organic compound is set to be equal to 0.1 ppm or lower (preferably, equal to 0.01 ppm or lower). The ionic impurity refers to an element belonging to Group I or II in the periodic table, and typically to sodium (Na) or potassium (K).
Accordingly, in order to obtain the above-mentioned structure, it is necessary to use such a light-emitting organic compound that contains ionic impurities at the concentration of 0.1 ppm or lower (preferably, at the concentration of 0.01 ppm or lower).
In the case of sodium, the above-mentioned concentration range can be calculated to be 7xc3x971017 atoms/cm3 or lower (preferably, 7xc3x971016 atoms/cm3 or lower). However, it is appropriate to consider that the total concentration of all of the ionic impurities should meet the above-mentioned concentration range.
When a light-emitting organic compound made of a low-molecular compound (hereinafter, referred to as the low-molecular type EL compound) is used for obtaining the above-mentioned light-emitting organic compound, the low-molecular type EL compound can be purified by a zone purification method, a sublimation purification method, a recrystallization method, a distillation method, a filtration method, a column chromatography method, or a reprecipitation method.
On the other hand, when a light-emitting organic compound made of a high-molecular compound (hereinafter, referred to as the high-molecular type EL compound) is used, values of molecular weight are likely to vary over a certain range since degree of polymerization cannot be completely controlled. Thus, a melting temperature of the resultant material cannot be decided unambiguously at a certain value, and therefore, it becomes difficult to perform purification. In this case, it is appropriate to perform a dialysis method or a high-performance liquid chromatography method. In particular, it is appropriate to perform an electrodialysis method for efficiently eliminating ionic impurities in the dialysis method.
In either of the above-mentioned purification methods, a purification process is required to be repeated several times in order to reduce the concentration of the ionic impurities to a level of 0.1 ppm or lower. More specifically, it is desirable to repeat a purification process at least three times or more, and more preferably, five times or more. Instead of repeating the same purification process, it is of course possible to perform two or more different processes.
In the case where the filtration method is employed, it is preferable to use a filter provided with openings having a diameter of 0.1 xcexcm (this diameter is particularly referred to as the diameter of particle-eliminating opening). Preferably, a filter with openings having a diameter of 0.05 xcexcm is used. A filter provided with openings having a diameter of 0.1 xcexcm only allows particles having a diameter of 0.1 xcexcm or smaller to pass therethrough. Similarly, a filter provided with openings having a diameter of 0.05 xcexcm only allows particles having a diameter of 0.05 xcexcm or smaller to pass therethrough.
As set forth above, in accordance with the present invention, a light-emitting organic compound containing ionic impurities at the concentration of 0.1 ppm or lower (preferably, at the concentration of 0.01 ppm or lower) is formed., and by using it, an EL device including a thin film made of a light-emitting organic compound having a volume resistivity in the range of 3xc3x971010 xcexa9cm or larger. A volume resistivity of a thin film made of a light-emitting organic compound in an EL device is set to be in the range of 1xc3x971011 to 1xc3x971012 cm (preferably, in the range from 1xc3x971012 to 1xc3x971013 xcexa9cm) is formed so as to fabricate an EL display device by utilizing such an EL device.
For the light-emitting organic compound to be used in the present invention. as the low-molecular type EL compound, a compound having a molecular weight in the range of 1xc3x97102 to 8xc3x97102 g/mol (typically, in the range of 3xc3x97102 to 5xc3x97102 g/mol) can be used, while a compound having a molecular weight in the range of 8xc3x97102 to 2xc3x97106 g/mol (typically, in the range of 1xc3x97104 to 1xc3x97105 g/mol) can be used as the high-molecular type EL compound.
The typical low-molecular type EL compounds that can be used in the present invention include Alq3 (tris-8-quinolinolato aluminum complex). Its molecular formula can be expressed as per enclosure.
[Formula 1]
The other possible compounds include distyl allylene amine derivative that can be obtained by adding amino-substituted DSA to DSA (distyl allylene derivative). DSA can be expressed by the Formula 2 in the separate sheet.
[Formula 2]
The typical high-molecular type EL compounds that can be used in the present invention include PPV (polyphenylenevinylene), which includes various types. For example, the molecular formulas 3 and 4 in the separate sheet have been presented (in the article by H. Shenk, H. Becker, O. Gelsen, E. Kluge, W. Kreuder, and H. Spreitzer entitled xe2x80x9cPolymers for Light-emitting Diodesxe2x80x9d in Euro Display Proceedings 1999, pp.33-37).
[Formula 3]
[Formula 4]
Alternatively, polyphenylvinyl having a molecular formula as described in Japanese Patent Application Laid-Open No. Hei 10-92576, as shown in the separate sheet, can also be used.
[Formula 5]
[Formula 6]
Various methods can be employed for forming a thin film of the above-mentioned high-molecular type EL compounds. In particular, a spin coating method is preferred in view of simplicity in its process. More specifically, in the spin coating method, a solute which forms a thin film is dissolved in a solvent and the obtained solution is applied to an underlying member by means of a spinner or the like. Thereafter, the solvent is volatilized in a baking process to form a thin film.
In accordance with the present invention, a solvent containing a high-molecular type EL compound is applied by means of a spinner, and a heat treatment is then performed at a temperature that is sufficiently low for preventing crystallization of the high-molecular type EL compound (specifically, at a glass-transition temperature or lower) so as to volatilize the solvent. As a result, a thin film made of the high-molecular type EL compound can be formed on the substrate.
Furthermore, since a light-emitting organic compound is vulnerable to oxygen a conductive film to be formed following formation of the thin film made of the light-emitting organic compound is desirably formed in such a condition that the thin film made of the light-emitting organic compound is not exposed to surrounding atmosphere containing water and/or oxygen. Accordingly, it can be preferable to form both the thin film made of the light-emitting organic compound and the conductive film to function as a cathode or an anode in the same thin-film formation apparatus.
For meeting the above-mentioned requirement, a thin-film formation apparatus of the multi-chamber type is suitable. In the present invention, it is preferable to form an EL display device having high reliability by utilizing such a thin-film formation apparatus.
With the above-mentioned structure, a current that is caused by reasons other than the carrier recombination can be prevented from flowing through a thin film made of the light-emitting organic compound that is contained in an EL device, and deterioration caused by unnecessary heat generation can be prevented. Accordingly, it is possible to obtain an EL display device with high reliability. Moreover, an electronic apparatus with a highly reliable display portion can be obtained by utilizing such an EL display device as its display portion.